Abstract
Using 1H and 13C NMR spectroscopy, the interaction of tetrapropoxy-p-tert-butyl-calix[4]arene (1) with H3O+ ions produced by hydrogen bis(1,2-dicarbollyl) cobaltate (HDCC) and traces of water was studied in nitrobenzene-d 5. It was shown that 1 readily forms an equimolecular complex with H3O+. The equilibrium constant K of its formation is 2.6 at 296 K. Exchange between bound and free 1 is fast even under mild excess of HDCC, the correlation time τex being about 0.13 ms. NMR shows that H3O+ is bound to the aryl-oxygen atoms and this binding forces the calixarene cup to adopt a more open and symmetrical conformation. This conclusion is in full accord with high precision quantum DFT calculations which find one structure of the complex corresponding to a global energy minimum, in which the H3O+ ion is bound to three of the oxygen atoms by strong hydrogen bonds and to the remaining oxygen by two weaker hydrogen bonds. The calixarene part is forced into a C4 symmetrical opened form.
When stored for weeks, the complex gradually transforms into other forms, most probably its hydrates, according to spectral evidence and DFT calculations.
Acknowledgements
This work was supported by the Academy of Sciences of the Czech Republic, Project T400500402, and the Czech Ministry of Education, Youth and Sports, Projects MSM 4977751303 and MSM 6046137307.